The present invention relates to a PABX cordless telephone system and particularly to the security arrangements for such a system.
A typical domestic cordless telephone system comprises a base station which is connected to the public switching network and a handset which communicates with its associated base station by a duplex radio link. The handset includes a keypad so that the outgoing calls can be made via the radio link to the base station and then onwards through the public switching network. Conversely calls on the public switching network for a handset are relayed by the radio link from the base station which includes the necessary control circuitry. As the number of cordless telephone systems exceeds the number of radio channels which can be allocated then there is a risk of conflict between users. Perhaps the most serious conflict is third party who has a handset making an outgoing call via a first party's base station due to the fact that both parties have handsets which operate on the same radio channel.
This conflict and other conflicts are discussed in European Patent Specification No. 0 196 834 which also describes providing each cordless telephone system with a security arrangement whereby each base station/handset system has a multiple bit address and this address forms a part of the signalling protocol in setting up the radio link between the base station and the handset. This specification describes how the multiple bit address and the channel number can be changed automatically when the handset is placed in a cradle provided on the base station for the recharging of a rechargeable battery in the handset. This address and number changing is done by a pseudo-random number generator which is cycled so that each new number is stored successively in memories provided in both the base station and handset. Acknowledgement of the receipt of the data by the handset is done by echoing back the same data to the base station via the charge contact interface. When the battery is recharged then as part of the operation of removing the handset from the cradle in the base station, the most recent (security) address and channel number are maintained uncharged in both memories. Such an arrangement provides a high degree of security.
In a PABX system the situation is different from a domestic system in that the base station has to be able to communicate via respective radio channels with a plurality of handsets. Within the system one wants each handset to be able to make and receive calls by way of the public switching network as well as to have intercom calls from one handset to another by way of the base station. In consequence part of the control function of the base station is to check continuously which channels and handsets are free. Additionally as part of the security arrangement, a multiple bit address, say a 16 bit address, of each handset in the PABX system can be altered in the manner described in European Patent Specification No. 0 196 834 when placed in respective battery charging sockets. A memory in each handset stores its respective address and channel. However a memory in the base station has to be store the latest number of each handset and the channel allocated. One method of avoiding conflicts between the handsets on neighbouring PABX systems would be to address each handset in turn and obtain an appropriate acknowledgement, for example busy or free. A typical signalling rate is 115 bits/second and assuming that there are 10 handsets and the signalling protocol comprised a 25-bit code then including allowing time, say 20 ms, for an acknowledgement from each handset, a minimum of 3.2 seconds would be required to execute one sequence, which is unacceptable.